1. Field of the Invention
The present invention relates to an image forming apparatus that performs light exposure using a light source that has a plurality of light-emitting portions.
2. Description of the Related Art
Conventionally, semiconductor lasers that can perform direct intensity modulation by drive current have been employed in optical scanning apparatuses that are mounted in image forming apparatuses. It is known that there is a significant difference with respect to the drive current/light output characteristics between each light emitting device. Also, it is known that even for the same light emitting device, these characteristics vary according to the environmental temperature or the device temperature. Therefore, APC (auto power control) is employed. In APC, the light power of a semiconductor laser is monitored and light power control is carried out so that the light power keeps a target value.
A multi-beam optical system is also known. The system can form a plurality of lines with one scan by using a plurality of light sources. A method has been proposed that performs light power control of each light source in a multi-beam optical system by causing all the light sources provided in the multi-beam optical system to emit light in a sequential order in a non-image period within one scanning cycle (Japanese Patent Application Laid-open No. 7-12709).
A method has also been proposed that feeds a bias current of a level that does not emit light to a laser in order to improve the responsiveness of the laser (Japanese Patent No. 3255295). In order to decide this bias current, light power control is required for each light source.
The number of light sources used in multi-beam optical systems tends to increase as the result of demands for faster print speeds and higher image quality in recent years. For the same reasons, the number of mirror planes of rotating polygonal mirrors is also increasing and the rotating speed is being sped up. As a result, a non-image period included in a single scanning cycle is becoming shorter than heretofore.
On the other hand, it is necessary to execute not only APC for adjusting the light power when the light is on but also APC for adjusting the bias current for each light source in the non-image period. However, since a period of from several microseconds to several tens of microseconds is generally required as the execution time for these APCs, the shorter the non-image period becomes, the more difficult it is to accurately execute the APC. In particular, it is becoming difficult to execute APC in a single non-image period for all light sources comprised by an optical system.